During the combusting of liquid or gaseous fuels in the combustor of a gas turbine, the so-called lean premix combustion has been established. In this, the fuel and the combustion air are premixed as evenly as possible and then directed into the combustor. For ecological reasons, attention is paid to a low flame temperature which is achieved as a result of a large surplus of air (“lean premix”). In this way, the development of nitrogen oxide can be kept low. Corresponding premix burners, which are also known as EV burners (EV stands for Environmental Vortex), are described for example in EP 321 809. Burners of this type are used in annular combustors or silo combustors. The schematic construction of silo combustors with EV burners is reproduced for example in FIG. 2 of DE 10 2005 062 284.
In combustors of this type, as a result of a mutual build-up of thermal and acoustic disturbances, thermoacoustic vibrations occur, as is known, which are not only loud but can adopt large vibration amplitudes in such a way that the gas turbine borders on the limit of its mechanical loadability and permanent damage may occur. In order to prevent it, in today's combustors provision is made for so-called Helmholtz dampers with which the possible vibration amplitudes in the combustor are lessened or even eliminated.
Since the thermoacoustic vibrations which occur in a combustor are influenced in frequency and amplitude by the extremely varied geometric and operating parameters of the combustor, in the case of a new combustor the anticipated vibrations can only be very unsatisfactorily predicted. It can be, therefore, that the Helmholtz dampers which are used on the combustor are initially not optimally matched to the vibrations which actually occur in the combustor.
It is discussed in the aforesaid DE 10 2005 062 284, in the case of an annular combustor or silo combustor, to connect correspondingly designed Helmholtz dampers to the combustion chamber of the combustor in place of individual EV burners. The individual Helmholtz dampers in this case may comprise a plurality of damping volumes connected in series. Furthermore, provision may be made for an adjusting mechanism with which at least one of the damping volumes can be steplessly altered from outside. For the operation of the adjusting mechanism, provision is made for a piston rod (37 in FIG. 2 of DE 10 2005 062 284), for which a suitable bushing has to be arranged in the associated housing opening (26 in FIG. 2). Also, apart from the adjusting mechanism, the damping volume cannot be altered from its basic structure, which impairs the flexibility of the Helmholtz dampers in use.
An annular combustor for a gas turbine, which is equipped with EV burners, is known from WO 03/060381. In this case, a Helmholtz damper, which comprises a constant and a variable volume, is installed between adjacent burners. For adjustment of the variable volume, provision is made for a special closable access in the turbine housing, through which the adjustment can be carried out by means of an insertable tool.